1. Field of the Invention
This invention relates to an image forming apparatus that performs printing on a sheet based on image data, and particularly, to an image forming apparatus, an image forming system, and an image forming method including a single-sided print mode and a double-sided print mode.
2. Description of the Related Art
In an electro photographic type image forming apparatus, a latent image is formed on a photoreceptor based on image data, and the latent image is developed by a toner image. The developed toner image is transferred to a sheet through a transfer unit. In an apparatus that temporarily transfers the toner image to an intermediate transfer unit, the toner image of the intermediate transfer unit is transferred to a sheet through a transfer unit equivalent to a secondary transfer unit.
In these transfer units, a positive charge is provided to the back side of the sheet to attract a negatively charged toner image to the sheet to transfer the toner image in order to ensure the transfer. A fixing apparatus then fixes the toner image to the sheet and discharges the sheet with the fixed toner image to a discharge tray.
An example of such an image forming apparatus includes an apparatus including two modes, a discharge mode of discharging the sheet provided with the fixed toner image without changing the direction of the front and back of the sheet and a discharge mode of discharging the sheet by inverting the front and back of the sheet (for example, see Japanese Patent Laid-Open No. 2007-168116). In such an image forming apparatus, the discharge modes can be switched for each job to facilitate separation on a job-to-job basis.
In the image forming apparatus, the positive charge is provided from the back side of the sheet as described above in the process of transferring the toner image to the sheet. Consequently, the surface provided with the toner image is negatively charged, and the surface on the opposite side provided with the positive charge is positively charged in the sheet after the transfer of the toner image.
When the charged sheets are discharged to the discharge tray as described above, if, for example, the directions of the front and back of the previous and next sheets between jobs are the same as the previously charged sheet as shown in FIG. 11, the positively charged surface and the negatively charged surface of the sheets are placed on top of each other. There is a problem that the sheets stick to each other or that it is difficult to put the sheets in order. The problem becomes more apparent when post-processing is applied to the sheets. The problem becomes particularly prominent in a printed matter in a PP (Production Print) device field.
In the image forming apparatus described in Japanese Patent Laid-Open No. 2007-168116, two modes of a mode of discharging the sheet without changing the direction of the front and back of the sheet and a mode of discharging the sheet by inverting the front and back of the sheet, are switched in each job.
In view of the problem, an apparatus including a discharge blower that discharges a sheet after transfer of an image is proposed (see Japanese Patent Laid-Open No. 10-181969). An apparatus including a first discharger and a second discharger to ensure the discharge is also proposed (see Japanese Patent Laid-Open No. 2004-10240).
However, in these image forming apparatuses, there is a possibility of attracting and attaching a positively charged surface and a negatively charged surface of sheets when the positively charged surface and the negatively charged surface on the front and back of the sheets are different at a copy-based breakpoint during double-sided printing, single-sided printing, or the like.
The present invention has been made in view of the circumstances, and at least an object of the present invention is to provide an image forming apparatus, an image forming system, and an image forming method that can take into account the polarity of the front and back of a preceding sheet at a breakpoint of jobs to prevent sticking of charged sheets.